Nickel alloys 600, 625, C-22 and A286: designation basics

Alloy 600, Alloy 625, C-22 and A-286 often appear together in specialty-steel enquiries. They should not be treated as one interchangeable “nickel alloy” group.

The first three are nickel-rich corrosion/high-temperature alloys. A-286 is different: it is an iron-base, nickel-chromium precipitation-hardening superalloy/stainless grade used primarily where high strength at elevated temperature is required.

Quick designation map

The correct purchasing specification still depends on product form: plate, sheet, bar, forging, pipe, tube, wire or fastener.

Alloy 600 — UNS N06600 / 2.4816

Alloy 600 is a nickel-chromium alloy known for:

• high-temperature oxidation resistance;
• resistance in several caustic and high-purity-water environments;
• resistance to chloride-ion stress-corrosion cracking relative to many austenitic stainless steels;
• use in furnace, chemical-processing and nuclear-related applications.

Special Metals lists UNS N06600 / W.Nr. 2.4816 for INCONEL® alloy 600.

“INCONEL” is a trademark. On a technical purchase order, the generic UNS/material designation and product standard should be clear rather than relying only on a trade name.

Alloy 625 — UNS N06625 / 2.4856

Alloy 625 is a Ni-Cr-Mo alloy strengthened significantly by molybdenum and niobium additions.

Its combination of strength, fabricability and corrosion resistance makes it common in:

• seawater/offshore components;
• chemical processing;
• high-temperature equipment;
• bellows and expansion joints;
• piping/tubing and weld-overlay applications.

Special Metals lists UNS N06625 / W.Nr. 2.4856.

Alloy C-22 — UNS N06022 / 2.4602

C-22 is a highly alloyed Ni-Cr-Mo-W corrosion-resistant alloy designed for severe mixed chemical environments.

Haynes lists HASTELLOY® C-22® as UNS N06022 and identifies 2.4602 in the European/DIN material-number context.

The high chromium, molybdenum and tungsten content gives the alloy broad resistance to oxidizing and reducing corrosive media.

Again, “HASTELLOY” is a trademark. The technical purchase description should state the grade/UNS and product specification.

A-286 is the outlier

A-286 is often grouped commercially with nickel/superalloy products because it contains about 24–27% Ni and is sold by specialty-alloy producers.

Metallurgically, however, it is an iron-base austenitic precipitation-hardening alloy.

Common designations include:

• UNS S66286;
• 1.4980;
• X6NiCrTiMoVB25-15-2;
• AISI/Type 660 in some specification contexts.

ATI and Böhler describe A-286 as precipitation hardenable and used for high-temperature high-strength components such as turbine parts and fasteners.

This is a very different design objective from selecting Alloy 625 or C-22 mainly for severe corrosion resistance.

Do not compare these alloys only by nickel content

Nickel percentage alone does not tell you what the alloy is designed to do.

Compare:

• chromium, molybdenum, niobium, tungsten, titanium and other strengthening/stabilizing elements;
• heat-treatment/age-hardening condition;
• corrosion mechanism;
• high-temperature strength and creep requirements;
• product standard;
• weldability and fabrication route;
• sour-service or project qualification.

For the general method, see How to compare steel grades across standards.

Sour-service qualification is separate

Some product forms of Alloy 625, C-22 or A-286 may appear in NACE/ISO 15156 applications, but acceptance depends on the material condition and environmental limits in the governing standard.

See NACE MR0175 / ISO 15156 applicability.

Heat treatment can be part of the grade definition

A-286 in particular may be supplied solution annealed and then precipitation/age hardened to develop strength.

Do not confuse this with ordinary stainless solution annealing as the final property condition. See Solution annealing of stainless and duplex steel for the broader heat-treatment concept.

MetalMate takeaway

Alloy 600, 625 and C-22 are nickel-rich corrosion/high-temperature alloys; A-286 is an iron-base precipitation-hardening superalloy. Use UNS/material numbers plus the correct product standard, and do not treat trade names or nickel content as proof of equivalence.

Related Knowledge

• How to compare steel grades across standards
• NACE MR0175 / ISO 15156 applicability
• Solution annealing

References

• Special Metals, INCONEL alloy technical bulletins — Alloy 600 (N06600/2.4816) and Alloy 625 (N06625/2.4856): https://www.specialmetals.com/documents/technical-bulletins/
• Special Metals, INCONEL alloy 625 technical bulletin: https://www.specialmetals.com/documents/technical-bulletins/inconel/inconel-alloy-625.pdf
• Haynes International, HASTELLOY C-22 — UNS N06022 / 2.4602 context: https://haynesintl.com/en/alloys/alloy-portfolio/corrosion-resistant-alloys/hastelloy-c-22/
• ATI, A286 — UNS S66286: https://www.atimaterials.com/Products/Pages/ati-a286.aspx
• BÖHLER, T200 / Alloy 286 — 1.4980 / S66286: https://www.bohler-edelstahl.com/en/products/t200/